Studies will be performed to determine the changes in neutral amino acid transport systems of brain slices as donor rats mature over the first 3 weeks after birth; and the system(s) that are altered by thyroxine hormone. Present results show a change in the transport systems used by the model amino acid, alpha-aminoisobutyric acid, as the brain develops over this period. Thyroxine also increases the transport. We will identify the systems altered by age and by thyroxine, using amino acids transported extensively by each of the different neutral amino acid transport systems in brain (e.g., N-methyl alpha-aminoisobutyric acid, alanine, leucine). Rat diaphragm will be used in vitro to study the interrelationships among the actions of insulin, glucagon, and cyclic adenosinemonophosphate on amino acid transport. Present results show that insulin and glucagon increase transport of amino acids expected to use transport system A; while the cyclic nucleotide stimulates only some of these amino acids (proline, cycloleucine). The transport systems altered by the hormones will be identified. Analyses will also be made of the changes in cyclic AMP levels, and adenyl cyclase activity, under the various conditions, to determine the role of (if any) these factors play in the transport changes.